Optos are usually all or none but make sure it's not working (use your phone camera unless you have something better). Make sure transmitter and receiver opto lenses are clean. Check for a bad solder connection. I've never had an intermittent opto problem, only transmitter (projector) burn out. I think the transmitter is on the right side and receiver on the left on BSD.

One more idea-there are 2 wires to the opto transmitter on the right side. Use your meter and test the voltage here and start the machine from cold. You should see good DC steady voltage here from start up. If the voltage is not there but suddenly jumps up later you might have a bad capacitor on the driver board. If you have good, steady voltage from start up but the opto is stubborn just replace the opto.